Last year I received an email informing me that when someone went to 'pick' an item to put in my trunk they found that it wasn't there. They gave me a refund instead. This was before I had decided to ship anything in my trunk. This indicates to me that items do get 'picked' and placed in a 'trunk' in advance of any decision to ship, probably using spare time between normal 'picking'. This would save time overall as it means items in your trunk are ready to ship straight away. It also prevents those items being accidentally picked by someone else.
I expect this is all managed by a clever computer system, with the flesh-bots doing what it says, including stocktaking.